<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div {
      width: 100px;
      height: 100px;
      border: 1px solid red;
    }
  </style>
</head>
<body>
  <div class="box"></div>

  <script>
    var div = document.querySelector('div')

    div.classList.add('test')
    div.classList.remove('box')

    div.onclick = function() {
      this.classList.toggle('flag')
    }
  </script>
</body>
</html>